1. Field of the Invention
This invention relates to ultra-dense large scale integration of microelectronic components in integrated circuits (ICs), and more particularly to structures for distributing power to such circuits using a grid assembly attached to the top surface of the IC die and wire bonding down from the grid to the IC die surface.
2. Description of Related Art
Technological improvements in the electronics industry have made larger, more highly integrated ICs possible, and correspondingly, increased the demands upon the IC's power distribution system. Improved production methods allow the die size to increase and the feature size to decrease. The smaller feature size allows for more densely placing the microelectronic components on the die, and also allows for a lower operating voltage. By more highly integrating these components, VHSIC devices are achieved.
The demands on the distribution system for the improved ICs include: increased power demand per unit area of IC since more components reside in a unit area; longer path lengths for the current to traverse to reach the interior of the IC; greater sensitivity to drops in voltage due to the lower operating voltage of some types of ICs; greater demand for available die area for signal lines to interconnect the larger number of components; greater demand for Input/Output (I/0) pins on the IC package since there are more components to send or receive signals outside the IC.
Prior art power distribution systems can be inadequate because they typically provide power to the IC from the IC's periphery. Metallized areas on the IC, typically thin film metallization, bring current from the periphery to the interior of the IC. However, these metallized areas have a finite resistivity, and therefore, the voltage drops as the current flows toward the interior of the IC. In the large ICs, this voltage drop can exceed the operating requirements of the IC because the path length is longer and therefore the voltage drop greater. Moreover, this problem becomes more acute as the IC's operating voltage decreases. For example, some new CMOS devices are designed to operate at 3 volts instead of the typical 5 volts. A given voltage drop represents a larger percentage of the operating voltage in the 3 volt device, and hence, the 3 volt device cannot tolerate as large a voltage drop as a 5 volt device. In addition, the metallized areas for providing power to the IC use up valuable area that could be more advantageously used for more components and/or interconnect wiring.
Prior art solutions to these problems include increasing the metallized areas that bring power to the interior of the IC to reduce current density per unit length, and therefore, voltage drop per unit length. This solution is undesirable because it uses up even more valuable area; moreover, it increases the mean distance between components and therefore the mean propagation time between components.
In some multichip module applications, wire bonding to a ceramic substrate containing power busses is used to bring power to the ICs. The same problems arise as the multichip modules get larger. Valuable area is used for power busses that could otherwise be used for interconnects, making increases in the area used for power busses (in an effort to reduce voltage drop) undesirable. If the multichip module contains the large ICs described above, the problem is even further compounded. This solution appears to be practical only for small ICs and multichip modules.
Another prior art solution is disclosed in U.S. Pat. No. 4,612,564. Moyer describes an IC package that incorporates a leaded frame with cross-under members for power distribution. Bonds are made from the underside of the IC to the frame and cross-under members. This package uses the valuable area on the underside of the IC die for power distribution. As is well known, large ICs use their undersides in conjunction with standard packages to access I/O pins, making Moyer's IC package undesirable for these applications.
The present invention is directed to an improved power distribution system for ICs that overcome the limitations or problems of the prior art power distribution systems.